Risk associated with making a hole in a wall

[richard4556]

Hi. I'm curious about whether there is any unacceptable risk associated with making a rectangular hole in an external house wall.

In the picture, the wall is a no fines wall. That is, its made off pebble aggregates (20mm dimmer) coated in cement. There are many voids. It's a wall developed by the construction firm Wimpey in the UK. So, it's type of concrete wall, but not solid throughout. The wall is 340mm thick.

You see that there is a lintel over a length of 1800mm which covers a door way and a window opening. Note that the back wall to the left of the left edge of the door way goes for about 30 feet to the left, and the wall, to the right of the right side of the window opening goes about 20 feet to the right. Effectively we are looking at a doorway and a window opening in a 56 feet long wall.

There are two rectangular holes marked on the wall, each 200mm long by 55mm deep that go right through the wall. The bottom hole is hole "B" and it's at 60mm from the floor level. Now, if I make a hole in this position I have no worries about risking any structural problem. Now, hole "A" is in a position that is 30mm from the edge of the window opening right edge. The question is, would making that hole present a risk? My intention is to make a metal box of 10mm thick steel to fit in the rectangular hole, but is there a risk associated with making the hole, before I get to put in the steel box?

Hole "A" is almost tantamount to taking a notch out of the wall 250mm long, where there is a gap on the left of that notch( i.e the window opening).

I'm not asking for any calculations or any approval to make hole "A". I'm just seeking a knowledgeable person to express an opinion about whether making hole "A" represents a risk that should not be taken. I could probably make the hole in two goes, that is doing the right half first, putting in a supporting box, then doing the left half and putting in another supporting box. I do realize that no one may wish to give an opinion. Thanks.

 

[racookpe1978]

"exterior house wall" ... But we do not (nor do you!) know the loads across the top of the wall.

That is a "must know".

Will there be a structural member vertical between floor and the lintel between the door and window? Not just a door trim or window trim and water-proofing, but afull length structural member that the trim and flashing will attach to?

 

[dik]

Can you through bolt a couple of 'C' sections with the back to the wall to act as a 'needle beam'. The beam would have to be constructed high enough to provide clearance for the lintel. Cut out the wall as required beneath the needle beam leaving pockets for the lintel bearing and cast or place your lintel beam and provide the necessary openings...

I noted in an earlier posting that changing the load regime of an existing structure can have some unanticipated effects and the owner should be advised that some cracking of the existing wall above the opening may occur.

Dik

 

[richard4556]

Another way of describing this is as follows:

A wall of 340mm thickness is 1706Cm long, 5181mm high. Near the middle of that wall there is a rectangular opening that is 1800mm long in the horizontal and 2050mm in the vertical. Atop which is a lintel.

If you cut a rectangular notch into that wall, starting at the right edge of the opening, that is 250mm in horizontal length and 85mm in vertical length, the bottom of that notch at say 1030mm from the floor - would that notch present some sort of structural risk?

This in a case where the top of the wall is holding up the edge of a roof? Force on top of wall unknown.

( No support of the lintel in the middle, only the ends.)

Strictly, I'm not seeking to cut a notch. At any rate, I think in this case, the risk is a function of how I go about making the hole. As has been alluded to. It's how I go about making that hole. In which I may need instructing. Will look at "Needle beam" and see what gives. Any other suggestions welcome.

 

[dik]

If you are not sure of what you are doing, you should hire someone that has done this work; it is not for 'first timers'. There are load issues, existing arching action, foundation, etc.

What is the existing foundation? How are you impacting them? You could be overloading portions of it and if supported by grade beams you could be causing portions of it to crack and fail, in flexure or shear.

If the existing wall is on piles, there could be 'arching action' occurring between the piles, with little or no loading going to the grade beams. What lateral support is provided to or by the wall? You need more information.

Once the needle beam is correctly installed, it's just a matter of carefully removing portions of the wall as required. It may be a matter of providing some temporary lateral support of the remaining wall, etc. The openinc can be made by sawcutting or removal of the CMU units.

I've attached a design development sketch that I've used with other works; the sketch is illustrative only and very preliminary.

The reason for the pockets noted in my first post was to provide bearing support for the lintel, steel or concrete...

Dik

 

[richard4556]

Hi. Can I check understanding here: The opening in the back wall (the wall), for the door and the window, with a lintel above it - is already there. And the new hole I am seeking to make is a rectangular one of size 220mm in the horizontal by 85mm in the vertical - just to to the right of the opening. In the picture below, the only alteration I am making is making hole "A". Which is a small hole in a large wall. But it is close the the right edge of the big opening (which is already there), and the right hand side of a lintel.

EDIT: Given that there is a space under the lintel, I will (if required in the interests of reducing risk) be able to put under the lintel on the right hand side a prop (An acro prop here in the UK). That prop can be tightened to exert an amount of upward pressure on the lintel. So, when I create that 220mm x 85mm hole there is some measure of support on the right hand side of the lintel. After the hole is made the intention is to put a steel box inside the rectangular hole made of 10mm thick steel plate. Then run pipes through the hole.

 

[richard4556]

Hi. Possibly coming to a conclusion here. I'm not a structural engineer, however I have made a drawing and I wish to provide some analysis. In the drawing below we see 15 positions where a notch or a hole is made in the wall below the lintel. The notch or hole is 250mm by 85mm, starting with hole #1 just above the floor level, up to notch #15 which is immediately below the lintel.

Now, I'm looking at this problem as a lintel support problem, perhaps that is good enough. My intention to create a hole may be considered as effectively cutting a notch at notch #5 position.

My analysis is this: Any hole above the shelf line is effectively a notch which leaves the lintel held up by a cantilever made by a section of the wall. Below that line there is no cantilever. Any hole made below the shelf line is relatively a small risk. The risk increases the higher it is up the wall. At # 15, a notch, there is no support for the lintel and the lintel falls. Now, at notch #5, the depth of the cantilever wall support is 850mm and the length of the cantilever is 250mm. This cantilever is shown by the darker lines in the drawing. The drawing also shows where I would put a prop, between the lintel and the shelf top surface.

Now, let me try to put words in your mouth and see if they can be accepted. These words are: "If you create a hole at notch # 5, place a prop in the position shown to be doubly sure that the lintel will not move. Do not just rely on the cantilever."

EDIT: It would probably be wise to install two props, sitting on a beam, so the load would be spread along the shelf top. That shelf in my case is actually the top of the concrete, that is the wooden shelf board is removed at present.

EDIT: I could play safer and create a hole in # 3 position, avoiding the creation of a cantilever.

 

[richard4556]

Assuming that I'll be taking an acceptable risk if I employ two props, both sitting on the wall that is below what is the window opening - there arises the question of the pressure to be exerted by the props on the lintel. Of course, what is needed is some pressure that effectively relieves some of the force on the cantilever. I suppose you turn the screw on the acro prop as tight as you can. That's not very scientific, but I suppose what is done in practice. Correct?

Also, if I put in a box made of 10mm thick mild steel into the new hole, I suppose that may need to be made to exert some upward pressure on the cantilever, before the props are removed. Would that be correct? Thanks.

 

[TehMightyEngineer]

The only correct answer here is you need to have a qualified person look at this. By your own sketch you have an unknown load on this. You have no way to ensure your support is adequate. Hire a engineer qualified to perform the work in your jurisdiction.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[richard4556]

Ok, understood about the need for a qualified person to look at this. Thanks. If it was not for the fact that the hole I am seeking to create is close to the edge of a window opening I would feel confident in creating the hole. I may put a hole in at point #2. That should present no real risk. It's after all, a small hole in a big wall.

 

[TehMightyEngineer]

I've lost track of how many "small holes" I've seen in brick walls for piping, HVAC, electrical, that have cracked or failed brick above the opening. Can it be done easily? Sometimes, yes. Can it screw up a wall? Yes.

Even if it doesn't crack when you make the opening, without proper detailing you could easily cause issues down the road. Buildings are not static.

If it is easy and risk free as you say then getting a qualified person to investigate will cost a minimal fee. It should also be a small fee to find out if it is easy or not. If an engineer looks at it and quotes you a high price to do the work then maybe it wasn't so easy and risk free as you thought.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[racookpe1978]

You still don't seem to understand the risk YOU are taking - against professional advice and judgement - in proceeding. That house, roof, that door and window assembly, and that floor/joist/sub-floor is at risk of failure. And the cost of repair, replacement, and reimbursement of any injured person (plus all penalties) is YOUR responsibility if you cut even "a little hole" without understanding the consequences.

 

[BAretired]

Hole "A" is almost tantamount to taking a notch out of the wall 250mm long, where there is a gap on the left of that notch( i.e the window opening).

I agree. The 30mm strip of wall should be regarded as having no strength particularly with 20mm diameter aggregate and many voids. Whether the wall can perform satisfactorily with the notch would depend on the lintel reaction and the shear strength and tensile strength of the wall. Without a lot more information about the loads and material properties, I would consider the cutting of Hole 'A' to be a risk not worth taking.

BA

 

[richard4556]

I've contacted a local structural engineer. See what he can do for me. Best course of action.